There are many techniques used in the analysis of water for contaminants. They include a variety of traditional but tedious "wet chemistry" procedures that can, in principle and with varying degrees of success, determine the kind and concentration of many kinds of solute species, both organic and inorganic. In addition there are a number of bulk property measurements that can indicate the presence of particular kinds of contaminants, but give only approximate information about their nature and amount. Such "symptomatic" tests include measurements of electrical conductivity, pH, refractive index, turbidity, specific gravity, "hardness" and the like.
Many of the above-noted techniques are widely used and give useful indices of water quality, but do not provide the kind of detailed information that society is increasingly demanding as it has learned about the physiological effects of specific species, such as lead and mercury. Even at trace levels, such species have a powerful impact on the quality of life and health. A comprehensive technique for determination of such species should provide, quickly and cheaply, information on the identity and concentration of all elements, and if possible their compounds, that are present, even at trace levels, in a water sample.
The most widely used method that comes anywhere near to approaching an ideal test is Atomic Absorption Analysis (AAA). It is carried out by injecting a water sample into a flame, electrical discharge or other heat source that is hot enough to vaporize all solute species and to decompose them into their component atoms. Radiation from a discharge lamp of a particular elemental species is then passed through the hot vapor. The attenuation of that radiation is a measure of the concentration of the lamp species in that vapor. A different lamp must be provided for each species that is to be determined. Even so, the method is relatively simple and inexpensive and can readily determine elemental concentrations in the water sample with sensitivities in the parts/million range.
A more sensitive method that is useful with a wider range of species is Inductively Coupled Plasma Mass Spectrometry or ICPMS. It also involves injecting a water sample into a high temperature plasma produced by an electrode-less discharge that decomposes essentially all solute species to their constituent atoms or very simple and stable compounds thereof. The discharge transforms some of each of these simple species into ions that are passed into a mass analyzer to produces a mass spectra. Such a spectra generally shows at least one peak for each element or stable compound present in the discharge. ICPMS is very general and much faster than AAA in that it can analyze for all elements, almost simultaneously, without the need of a separate source of radiation, i.e. a lamp, for each species, as is required by AAA. Moreover, it is much faster and more sensitive than AAA, being capable of measuring concentrations at the parts per billion level in the original water sample. It is also more costly because of its need for a mass spectrometer which is a relatively complex and expensive piece of equipment.
Mass spectrometry consists in "weighing" individual molecules by transforming them into ions in vacuo and measuring the response of their trajectories to various combinations of electric and/or magnetic fields. It follows that production of ions from a species of interest, for example by a discharge in ICPMS, is an essential step in performing mass spectrometric analysis. The present invention relates to this essential step, in particular to the relatively new method of producing ions known as "Electrospray Ionization" (ESI), first proposed by Malcolm Dole and co-workers in 1968 (J. Chem. Phys. 49, 2240). This unique approach to ionization, quite different in nature from the method used in ICPMS, did not receive much attention until 1984 when the first results of Yamashita and Fenn were published in America (J. Phys. Chem. 88, 4451 (1984). Similar results were published almost simultaneously in Russia by Gall' et al (Sov. Phys. Tech. Phys. 29, 911 (1984)). Since then the efforts of many investigators have developed ESI into a widely practiced technology that has been described at length in a number of U.S. patents (Labowsky et al, U.S. Pat. No. 4,531,056; Yamashita et al, U.S. Pat. No. 4,542,293; Henion et al, U.S. Pat. No. 4,861,988; and Smith et al, U.S. Pat. Nos. 4,842,701 and 4,885,706;Fenn et al U.S. Pat. No. 5,130,538) as well as in a number of review articles (Fenn et al, Science 246, 64 (1989); Fenn et al, Mass Spectrometry Reviews, 6, 37 (1990); Smith et al, Anal. Chem. 62, 882 (1990); Kebarle and Tang, ibid. 65, 972A (1993)).
ESI has greatly expanded the application of mass spectrometry to the analysis of species comprising solutes in liquid solutions. As an extremely effective species-identifying detector for liquid chromatography, Electrospray Mass Spectrometry (ESMS) is now in daily use by investigators throughout the world. Even when the analyte solute species comprise large, complex and fragile molecules, ESI can transform them, intact, into ions in vacuum ready for mass analysis. Moreover, the ions of large molecules are characterized by such extensive multiple charging that their mass/charge (m/z) ratios are rarely above about 2500, even when the molecular weight (Mr) of the parent molecule is as high as five million! (Nohmi and Fenn, J. Am. Chem. Soc. 114, 3241 (1992)). Consequently, they can be "weighed" with almost any available mass-analyzer. Even relatively inexpensive instruments such as quadrupole mass filters, ion traps, and time-of-flight machines generally have enough resolving power and mass range to obtain reliable values of Mr up to 100,000 or so with an accuracy of 0.01%. Equivalent or better accuracy can be achieved for much larger species by magnetic sector instruments and Fourier-Transform Ion Cyclotron Resonance (FTICR) machines.
Because of its unique ability to produce ions from very large molecules, most studies and applications of ESMS have thus far related to the relatively complex and fragile species involved in biological organisms and their metabolic processes. However, recent results indicate that ESMS might also be very effective in detecting and identifying other kinds of species. The subject invention relates to improvements in the ability of ESMS to detect, and to determine the identity and concentration of, small inorganic ions in water, even at the trace levels that are encountered in various kinds of environmental waters. Availability of a sensitive method of analyzing water for these small inorganic ions is important because even at very low levels, they can have a very large impact on many forms of life. This invention opens the way to substantial improvements in the effectiveness with which ESMS can detect and identify small but significant species. Practice of the invention makes ESMS able to analyze water for small inorganic ions at concentration levels as low as parts per trillion.